1. Field of the Invention
This invention, relating to fabrication methods for high-performance R--Fe--B permanent magnets with excellent crystal orientation, provides a fabrication method whereby cast and ground alloys of a desired composition obtained either by ingot grinding, Ca reduction diffusion or strip casting, are ground to a coarse and then a fine powder, and packed into a mold at a particular packing density, and whereby, after aligning the magnetic powders by repeatedly applying an instantaneous pulsed magnetic field to invert their magnetic orientation, they undergo cold isostatic pressing, sintering and aging. In particular, it relates to a fabrication method whereby a lubricant is compounded with the coarse powders before fine grinding and cold isostatic pressing is performed in a static magnetic field to obtain high-performance R--Fe--B permanent magnets with excellent orientation and magnetic characteristics such that iHc is greater than 10 kOe, and that the sum of A, the maximum energy product (BH)max(MGOe), which is one characteristic of a magnet, and B, the coercive force iHc(kOe), has a value A+B of more than 59.5.
2. Description of the Prior Art
Currently, good magnetic characteristics can be obtained for typical R--Fe--B permanent magnets used as high-performance permanent magnets (J.P.A. No. SHO-59-46008, U.S. Pat. No. 4,770,723), in compositions consisting of a ternary tetragonal compound as main phase and an R-rich phase, and R--Fe--B permanent magnets of various compositions are used in a wide range of products from general home appliances to computer peripherals, utilizing their many varied magnetic characteristics.
However, the drive for miniaturization and high performance in electrical device has meant a search for high performance and more inexpensive R--Fe--B permanent magnets.
In general, R--Fe--B rare-earth magnets are usually fabricated by either process 1).about.3) or process a).about.c).
1) For starting materials, fabricating a cast alloy by induction melting of rare-earth metals, electrolytic iron, ferroboron alloy and in addition, electrolytic Co. PA0 2) Forming coarse powders from this cast alloy by H.sub.2 absorption and decomposition, and then forming fine powders of 1.0 .mu.m.about.10 .mu.m either by wet grinding using a ball mill or attrition mill, or by grinding with a jet mill using an inert gas.(J.P.A. No. SHO-60-63304 SHO-63-33505) PA0 3) Pressing, sintering and aging the fine powder.
a) Using starting materials whereby a mixed oxide or alloy powder of a required composition is compounded from at least one rare-earth oxide, iron powder, and at least one of either pure boron powder, ferroboron powder or boron oxide, or is comprised of the above elements. This material is mixed with metallic Ca and CaCl.sub.2, and a reduction diffusion reaction is performed within an inert gas atmosphere. The resulting reaction product is slurrified, and the CaO by-products and CaCl.sub.2 flux are removed by a washing treatment. PA1 b) Wet grinding the resulting products in a ball mill or attrition mill, or dry grinding them in a jet mill to produce fine powders of 1.0 .mu.m.about.10 .mu.m. PA1 c) Pressing; sintering and aging the fine powder.
Further, fabrication methods have been proposed (J.P.A. No. SHO-63-317643) whereby, in order to prevent coarsification, residual .alpha.-Fe and segregation of R--Fe--B alloy powder crystallites with unavoidable defects formed by the ingot grinding method, that is, a method whereby ingots are pulverized and the resulting ground alloys are mechanically ground to a coarse powder followed by mechanical grinding or grinding in a jet mill, a R--Fe--B molten alloy is formed into a cast alloy of a particular thickness using the twin roller method. Then, following usual metallurgical methods, the cast alloy is ground to a coarse powder by a stamp mill or jaw crusher, and then to a fine powder of average size 3.about.5 .mu.m by a disk mill, ball mill, attrition mill or jet mill, and then finally pressed in a magnetic field, sintered and aged.
However, using the above method, we cannot achieve a rapid improvement in grinding efficiency compared to prior ingot grinding methods, where ingots were cast into molds, and further, as not only the particle surfaces but also the particle bulk is ground during the fine grinding, we cannot achieve a great improvement in magnetic properties. Also, as the R-rich phase does not form RH.sub.2, which is stable against oxidation, the large microscopic surface area of the R-rich phase being microscopic leads to a degradation of the antioxidation properties. As such, oxidation occurs during processing meaning and we cannot obtain good magnetic properties.
As greater cost efficiency is being sought in the production of R--Fe--B permanent magnets, it is necessary to efficiently fabricate raw material powders for high-performance permanent magnets. As such, it is necessary to improve fabrication conditions to produce near theoretical properties.
With the purpose of producing a fabrication method for high-efficiency R--Fe--B permanent magnets whereby, efficient fine grinding is possible to achieve a good iHc due to the fineness of magnetic crystallites with good antioxidation properties and whereby there exists a high degree of orientation of the magnetization direction of each crystallite such that the sum of A, the value of (BH)max (MGOe) and B, the value of iHc (kOe) is A+B.gtoreq.59, the authors have proposed a fabrication method (J.P.A. No. HEI-5-192886) for high-performance R--Fe--B permanent magnets whereby R--Fe--B-type cast alloys of a particular thickness obtained by strip casting are coarse ground by a H.sub.2 absorption decay method and then ground by, a jet mill within an inert gas atmosphere, and whereby, the obtained fine powders are packed into a mold at a particular packing density followed by orientation by applying a pulsed magnetic field in a particular direction, instantaneously followed by molding, sintering and an aging treatment.
However, with a purpose of raising the performance of R--Fe--B permanent magnets, in order to improve the packing characteristics within the mold and the degree of orientation,when, for example, the fine powders obtained by the above method are compounded with a lubricant before press molding, it is extremely difficult to uniformly coat the fine powder's surface with a lubricant, and furthermore, imperfections such as variations in weight and cracks during pressing process.